Modern vehicles which are powered by an internal combustion engine utilize a number of techniques and technologies in order to significantly reduce pollutant emissions. For example, so-called "tailpipe" emissions are typically reduced by: 1) monitoring the oxygen content of the exhaust gas (e.g., via an oxygen sensor mounted in the exhaust path of the engine) and adjusting the air to fuel ratio introduced into the engine; and 2) passing the exhaust gases of the internal combustion process through a catalytic converter prior to releasing them into the ambient atmosphere.
However, another manner in which internal combustion powered vehicles produce pollutants is through the evaporation of fuel, for example, from the fuel supply tank (or "gas tank") of the vehicle. Regulations continue to reduce the allowable limits of such "evaporative emissions". One widely used method of reducing evaporative emissions from vehicles is to provide a fuel vapor storage canister for storing evaporated fuel vapor. Such a canister normally includes a carbon-based component which traps and stores the hydrocarbon vapor. The canister is typically "purged" during engine operation, meaning that the fuel vapor stored in the canister is transferred to a fuel intake portion of the engine (e.g., the intake manifold or throttle body) and is thus internally combusted by the engine as a portion of its fuel consumption.
Determining exactly when the canister should be purged and at what rate can be problematic. Since any purging of the canister involves the supply of additional fuel to the engine, ideally, compensation should be made to the rate at which fuel is fed directly to the engine from the fuel tank. If the engine has been idle (e.g., following a startup of the engine after extended parking of the vehicle), it may be expected that the canister will have accumulated some degree of fuel vapor content. The vapor content of the canister is really determined, to a great extent, by the ambient conditions of the vehicle, such as ambient temperature, ambient pressure, particular fuel mixture, etc.
Following engine startup, the canister is normally purged to some degree. Purging is not, however, usually initiated immediately upon startup, but only after the engine has entered a "closed loop" mode of operation. The oxygen sensor which is located in the exhaust path and which is used, along with other factors, to determine the fuel forward feed ratio of the engine does not normally provide accurate readings until it has reached an elevated temperature. Before this point, the engine is typically operated in an "open loop" mode of operation, wherein the appropriate air to fuel ratio of the engine is approximated, e.g., by software provided as read only memory within an "Engine Control Module" (or "ECM"), which is typically a microprocessor.
Once the engine enters a closed loop mode of operation, where the readings from the oxygen sensor are considered sufficiently reliable, software algorithms are also normally employed to control the fuel flow to the engine based upon readings of the oxygen sensor located in the exhaust path of the engine.
During purging of the canister, fuel vapor stored therein is transferred to the fuel intake portion of the engine through a purge line connecting the canister to the engine. In many modern vehicles, flow through the purge line is controlled using a "purge solenoid" located in the purge line. The flow rate from the canister to the engine via the purge line is controlled through modulation of the purge solenoid by the ECM.
It is not only during initial engine startup that the canister is typically purged. Hydrocarbon vapor is continually being produced, to some degree, in the fuel tank and therefore stored in the canister, even during actual operation of the vehicle. As noted above, the rate of production of hydrocarbon vapor in the fuel tank is dependent upon environmental variables such as ambient temperature and pressure which can vary widely and rapidly. The rate of fuel vapor production also vary with respect to commercial fuel mixtures (e.g., summer, winter, etc.).
With allowable levels of evaporative emissions being reduced, the trend is toward increasing purge line flow in order to reduce evaporative emissions. A positive flow from the canister to the engine may exist during a majority of the time the vehicle is being operated.
The software algorithms discussed above attempt to determine an appropriate purge flow rate based upon variables such as ambient temperature, time of engine operation, etc. They attempt to estimate the fuel vapor content of the canister and appropriate purge flow rates and can be difficult to calibrate. That is, such software algorithms do not employ any real time determination of the actual fuel vapor content of the canister. Any adjustment which is made to the normal fuel feed rate to compensate for the fuel vapor fed to the engine through the purge line is therefore an approximation of actual conditions.
A continuous purging of the canister, or an over purging of the canister, can cause the canister to become contaminated, thereby reducing its efficiency and/or shortening its service life. A typical storage canister is provided with a fresh air port. During purging, air from the ambient environment enters the canister through the fresh air port replacing the flow of fuel vapor supplied to the engine through the purge line. Such "fresh air" can contain slush, salt, dirt, and all of the various contaminants to which modern vehicles are subjected. Such contaminants can seriously diminish the efficiency and service life of the canister.
The phenomenon of "canister breakthrough" is also a factor which can increase evaporative emissions from a vehicle. Depending upon environmental conditions and the size of a particular canister, a saturation of the canister may cause a spillover of fuel vapor into the ambient environment, and thus defeat the intended purpose of the canister.
In view of the above considerations, it will be appreciated that the purge flow should be sufficient to prevent canister breakthrough, an actual determination of the rate at which fuel vapor is being fed to the engine through the purge line allows for a more accurate compensation in the fuel equation of the engine, and the canister should not be excessively purged which can reduce its efficiency.